1. Field of the Invention
The present invention relates to an image reading apparatus and an image expansion and contraction correcting method that can be suitably implemented particularly in an operation of reading an image from a document that is being conveyed.
2. Description of the Related Art
Conventional image reading apparatuses that are mounted in copying machines and the likes include an image reading apparatus that reads image information from a document while moving the document with respect to a stationary image reading unit (such as a linear CCD). Referring to FIG. 20, an image forming apparatus equipped with a conventional image reading apparatus is described. FIG. 20 is a schematic view of the conventional image forming apparatus. In the following description, the “main scanning direction” generally means the direction perpendicular to the direction of conveying sheets (such as paper or film), and corresponds to the aligning direction of the photoelectric conversion elements of the linear CCD. Also, the direction perpendicular to the main scanning direction, which is the same as the sheet conveying direction, is the “sub-scanning direction”.
As shown in FIG. 20, the image forming apparatus includes an image reading apparatus A and an image recording apparatus B. The image reading apparatus A is located above the image recording apparatus B.
The structure of the image reading apparatus A is as follows. The image reading apparatus A includes a document reading unit 1 that reads an image from a document stationarily placed on a platen glass 22, and an automatic document feeder (ADF) 2 placed above the document reading unit 1.
The document reading unit 1 has an image reading unit 121 placed below the platen glass 22. The image reading unit 121 can be moved along a guide rail 24 by virtue of a driving unit (not shown). With the image reading unit 121 moving in the direction of the arrow F, an image can be read from the document stationarily placed on the platen glass 22. A reading unit 101 for reading an image from each document being conveyed from the ADF 2 is provided at a location adjacent to the platen glass 22.
The image reading unit 121 includes an illuminating unit 21c for illuminating each document, a linear CCD (Charge Coupled Device) 21g, and an optical system 21f that focuses reflected light from each document surface onto the linear CCD 21g. The light emitted from the illuminating unit 21c and reflected by a document surface is focused onto the linear CCD 21g by the optical system 21f, so that the image information of the surface of the document is photoelectrically converted.
In the ADF 2, documents D1 placed on a document tray 12 are separated from one another by separating units 44a and 44b, and are rolled out by a pick-up roller 43 as a feeding unit. Each document D1 is then conveyed to the reading unit 101 by a first conveying unit 146. The reading unit 121 reads image information from one side of the document at the reading unit 101. The document from which an image has been read is further conveyed by a second conveying unit 148, and is discharged onto a document discharge tray 50. In general, the first conveying unit 146 and the second conveying unit 148 are driven by a common driving source, so as to reduce the size of the apparatus.
In a case where an image is to be read from a document while the ADF 2 is conveying the document, the image reading unit 121 remains stationary at the reading unit 101, and reads an image from a surface of the document while the document is being conveyed.
In a case where an image is to be read from a document placed on the platen glass 22, the image reading unit 121 is moved to a position immediately below the platen glass 22. While the image reading unit 121 is being moved by a driving unit (not shown), an image is read from a surface of the document.
The image recording apparatus B is now described. In the image recording apparatus B, an optical image based on image information is emitted from a recording optical system 3 onto a photosensitive drum 10 that is an image bearing member, so as to form an electrostatic latent image on the drum surface. A developing unit 11 then develops the electrostatic latent image formed on the photosensitive drum 10 with a toner. Thus, a toner image is formed.
In synchronization with the formation of the toner image, a recording sheet P is supplied from a recording sheet feeder 4 that is provided at the bottom of the main body. The recording sheet P is conveyed by a conveying unit 5, and the toner image is transferred onto the recording sheet P by a transfer unit 6. The recording sheet P is conveyed to a fixing unit 7 to fix the transferred toner image. The recording sheet P is then discharged onto a discharge tray 9 by a discharging unit 8. Since the series of image forming procedures by an electrophotographic technique are well known, explanation of them is omitted herein.
Referring now to FIG. 21 and FIGS. 22A through 22D, data processing of each read image is described. FIG. 21 is a block diagram of the image processor of a conventional image reading unit. FIGS. 22A through 22D illustrate the charge transfer from the linear CCD.
As is well known, the linear CCD 21g has a number of photoelectric conversion elements G aligned in the main scanning direction. The photoelectric conversion elements constitute pixels G1, G2, G3, . . . Gn that read images. The linear CCD 21g is designed to read the data of each one line in the main scanning direction of each document, photoelectrically convert the data, and output the converted data as an image signal. When the linear CCD 21g receives light reflected by a document, charges representing the image information of the document are accumulated in the respective photoelectric conversion elements G (see FIG. 22A). Although all the charges accumulated in the reading pixels G1, G2, G3, . . . Gn are denoted by e1 in FIG. 22A, the charges accumulated in the respective reading pixels in practice have different charge amounts from one another in accordance with the image information of the document.
The charges accumulated in the respective reading pixels are collectively sent to a CCD analog register 71 (see FIG. 22B), and are then transferred to an analog signal processor 72 one by one by virtue of predetermined transfer pulses, while moving from one register cell to another neighboring register cell in the CCD analog register 71 (see FIG. 22C). After all the charges of the pixels are transferred to the analog signal processor 72, accumulated charges are collectively sent to the CCD analog register 71 (see FIG. 22D), and are transferred to the analog signal processor 72 one by one by virtue of predetermined clock pulses while moving from one register cell to another neighboring register cell (see FIG. 22C). This operation is repeated. Although all the charges accumulated in the reading pixels G1, G2, G3 . . . Gn are denoted by e2 in FIG. 22D, the charges accumulated in the respective reading pixels in practice have different charge amounts from one another in accordance with the image information of the document.
The analog signal processor 72 converts the transferred charges into an analog image signal by replacing the charges with voltage values. After various corrections such as a sensitivity correction are performed, an A-D converter 73 converts the analog image signal into a digital image signal. Further corrections are performed at a gain controller 74, a shading corrector 75, a tone controller 76, and the likes. The image data is then temporarily stored as the digital data of one line in a buffer memory 77. The image data is then read out to the image recording apparatus B.
When the transfer of the charges of one line to the CCD analog register 71 is completed in the above described operation, the image data of the next line is read in. However, through the above procedure, the document has moved in the sub-scanning direction with respect to the image reading unit 121, and the image of the second line is an image at a different location from the image of the first line in terms of the sub-scanning direction. The distance between the first line and the second line is predetermined in accordance with the reading resolution in the sub-scanning direction.
In the image recording apparatus B, image formation is performed on assumption that the read image data has been read at a predetermined reading resolution. Therefore, the CCD transfer pulse frequency, the number of transfer pulses, and the document conveying speed are determined so that each reading line interval (the sub-scanning interval) S becomes equal to a predetermined value in the document reading unit 1. Thus, image expansion and contraction are prevented in a recorded image.
If the document conveying speed becomes different from an ideal conveying speed due to a variation in the diameters of the rollers that are conveying a document in the above described structure, the reading line interval S in the sub-scanning direction becomes different from the predetermined value, resulting in image expansion and contraction. In such a case, the number of rotations of the driving motor that is driving the document conveying rollers is adjusted to optimize the document conveying speed and eliminate the read image expansion and contraction.
In recent years, image reading apparatuses that can read the image information from both sides of a document at once have been developed. Referring now to FIGS. 1 and 2, the structure of an image forming apparatus equipped with such an image reading apparatus is described.
As shown in FIG. 1, the image reading apparatus A includes a first image reading unit 21 and a second image reading unit 41. The first image reading unit 21 is to read one side of each document, having a first illuminating unit 21c that illuminates documents, an optical system 21f that is formed with mirrors 21d and an image focusing lens 21e, and a linear CCD 21g. The first image reading unit 21 is located below a platen glass 22 on which a document is placed. In the first image reading unit 21, the light emitted from the first illuminating unit 21c and reflected by a document face is focused onto the line CCD 21g to photoelectrically convert the information of the image on one side of the document.
The first image reading unit 21 can be moved by a driving unit (not shown), and can read an image from a document stationarily placed on the platen glass 22 while moving in the direction of the arrow F.
The second image reading unit 41 is to read the other side of each document, and includes a second illuminating unit 41c, an optical system 41f formed with mirrors 41d and an image focusing lens 41e, and a linear CCD 41g. The second image reading unit 41 is fixed at a predetermined location in an automatic document feeder (ADF) 40. In the second image reading unit 41, the light emitted from the second illuminating unit 41c and reflected by a document face is focused onto the CCD 41g by the optical system 41f, and the information of the image of the other side of the document is photoelectrically converted.
In the ADF 40, documents D1 stacked on a document tray 12 are separated from one another by separating units 44a and 44b, and are rolled out by a pick-up roller 43 serving as a feeding unit. Each document D1 is then conveyed to a first reading device by a first conveying unit 46. The first image reading unit 21 reads image information from one side of the document at the location of the first reading device. The document from which the image of one side has been read is further conveyed to a second reading device by a second conveying unit 48. The second image reading unit 41 reads image information from the other side of the document at the location of the second reading device. The document from which the image of the other side has also been read is discharged onto a document discharge tray 50 by a third conveying unit 51.
In a case where images are to be read from both sides of a document, the first image reading unit 21 is fixed to the first reading device. The document is conveyed by the ADF 40, and, during the conveyance, the first image reading unit 21 and the second image reading unit 41 read the images from both sides of the document.
In a case where a document placed on the platen glass 22 is to be read, the first image reading unit 21 is moved to a location immediately below the platen glass 22. The first image reading unit 21 then reads an image from one side of the document while being moved by a driving unit (not shown).
A technique related to the above is disclosed in Japanese Unexamined Patent Publication No. 11-258866 (Patent Document 1).
In an image reading apparatus, a document is conveyed by conveying units (or pairs of conveyance rollers). During the conveyance of a document, the combination of conveying units that are conveying the document changes in order. There are variations in roller diameter, rotation speed, and friction coefficient among the conveying units within the size tolerance of each component. Therefore, the document conveying speed is varied as the combination of conveying units changes. Theoretically, it is possible to prevent a change in document conveying speed by adjusting the conveying speed of each conveying unit. However, this is not a realistic approach, because the structure needs to become much more complicated so as to adjust the conveying speed of each conveying unit.
A change in document conveying speed during a document reading operation leads to expansion and contraction of the read image. Especially, in a structure having two image reading units for reading both sides of each document, the expansion and contraction of the image read from one side of a document differs from the expansion and contraction of the image read from the other side of the document. In the following, this problem is described in greater detail.
As described above, in the image reading apparatus that can read images from both sides of a document at once, the document conveying speed during a reading operation performed by the first image reading unit 21 is controlled mostly by the first conveying unit 46 and the second conveying unit 48. Meanwhile, the document conveying speed during a reading operation performed by the second image reading unit 41 is controlled mostly by the second conveying unit 48 and the third conveying unit 51. So as to eliminate a difference in image expansion and contraction between the front side and the back side of a document, the document conveying speed during the reading operation performed by the first image reading unit 21 and the document conveying speed during the reading operation performed by the second image reading unit 41 need to be made equal to each other.
However, it is difficult to have equal document conveying speeds among the first conveying unit 46, the second conveying unit 48, and the third conveying unit 51. For example, in a case where the first conveying unit 46, the second conveying unit 48, and the third conveying unit 51 are driven with the same driving source, there are slight differences in document conveying speeds, because the roller diameters of those conveying units are different from one another within the size tolerance or there are slight differences in pressure force among the pressure springs less than a given value for putting each pair of rollers into pressure contact with each other, even though the same pair of rollers are used for the first conveying unit 46, the second conveying unit 48, and the third conveying unit 51.
As a result, a difference in expansion and contraction rate in the sub-scanning direction is caused between the image read by the first image reading unit 21 and the image read by the second image reading unit 41. In the case where the conveying units are driven with the same driving motor, the difference in expansion and contraction rate between the two sides of a document cannot be eliminated by adjusting the number of rotations of the driving motor.
Alternatively, when an image is read from a document that is being conveyed, tension may be purposely applied to the document being conveyed. More specifically, the conveying speed of the conveying unit located on the downstream side of the reading position is made slightly higher than the conveying speed of the conveying unit located on the upstream side of the reading position, so as to apply tension to the document. With tension being applied to the document, the position of the document being read is secured in the sheet path, and image deformation and defocusing are prevented. However, since the conveying speed of the roller on the downstream side is higher in this case, a difference in expansion and contraction rate in the sub-scanning direction is caused between the image read by the first image reading unit 21 from one side of the document and the image read by the second image reading unit 41 from the other side of the document.